The phagocytosis-like process of engulfment is the hallmark of endospore formation in bacteria from the genera Bacillus and Clostridium, which produce unusually durable endospores that are the infectious agent of Anthrax and Botulism. Engulfment mediates a dramatic change in cellular architecture, rearranging the sporangium from two cells that lie side by side, to an endospore in which one cell (the forespore) lies within the cytoplasm of another (the mother cell). The studies supported by this grant have provided new insights into the mechanisms by which coordinated peptidoglycan synthesis and degradation mediate this dramatic example of the architectural plasticity of bacterial cells, and they are providing insight into the role essential proteins play in this process. We here propose to visualize the dramatic morphological rearrangements of sporulation and the protein complexes that mediate them, by using a new implementation of cryo-electron tomography that uses a focused ion beam to produce thin lamella of bacterial cells, producing images that reveal structures within cells at nanometer resolution. We also propose to pursue preliminary data that has revealed that sporulation entails a dramatic and previously unrecognized metabolic differentiation of the two cells, after which the future spore is completely dependent on the mother cell for the precursors for biosynthesis. This process, which appears to be mediated by a massive forespore-specific proteolysis event, effectively converts the two cells into synthrophic partners, providing an accessible model for studying coupled metabolism, which is prevalent in microbial communities and biofilms. We here propose to study these processes, using genetics, metabolomics, fluorescence and cryo- electron microscopy, integrating these experimental efforts with computational analysis and modeling. These studies will reveal the cellular and metabolic landscape of sporulation in unprecedented detail, capitalizing on the dispensability and streamlined machinery of sporulation to provide insight into conserved processes that are often essential for growth.